It is well known to build up an internal representation of a two-dimensional image to be displayed, in terms of discrete graphic segments that potentially overlap in the image. Where these graphic segments are to be displayed as solid bodies, then it is necessary to determine which segment has priority in the event of overlap between two segments. This depth priority issue is generally handled by assigning differing priorities to the various segments (either explicitly, or implicity through an hierarchical organization of the segments) and then resolving any overlap conflicts according to the assigned priorities of the segments concerned. The actual resolution process is effected as part of the overall process of converting the graphic segments into an output image.
A general discussion of image construction from multiple segments and of the priority resolution issue, can be found in most standard textbooks on graphics systems such as "Principles of Interactive Computer Graphics" William M. Newman and Robert F Sproull, second edition. McGraw-Hill.
When it is desired to update the output image, the previously existing collection of graphic segments is modified for example by the deletion or addition of one or more segments or by the alteration of the defining parameters of one or more segments. segments. The process of converting the segments into a new output image can then be carried out anew for the modified segment collection.
Reconstructing all of the output image is generally inefficient where only a small part of the image is effected by the modification of the segment collection. Accordingly, it is known to store an image representation and then restrict updating to a portion only of that representation, this portion being sufficient to encompass the affected area of the image. In one known system, the output image is nationally divided into an array of rectangular tiles delimiting update zones; whenever the underlying segment collection is modified, updating of the image representation is then restricted to a reconversion of the update zones that are affected by the modification. In another known system, updating of the image representation is restricted to a rectangular bounding box enclosing the affected image area. Both the foregoing approaches thus rely on artificial boundaries to delimit the update zone.
It is an object of the present invention to provide a graphics output system in which updating is bounded in a more natural manner.